Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the arterial blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heartbeat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmittance and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
Conventional pulse oximeters (also called contact PPG device herein) for measuring the heart rate and the (arterial) blood oxygen saturation (also called SpO2) of a subject are attached to the skin of the subject, for instance to a fingertip, earlobe or forehead. Therefore, they are referred to as ‘contact’ PPG devices. A typical pulse oximeter comprises a red LED and an infrared LED as light sources and one photodiode for detecting light that has been transmitted through patient tissue. Commercially available pulse oximeters quickly switch between measurements at a red and an infrared wavelength and thereby measure the transmittance of the same area or volume of tissue at two different wavelengths. This is referred to as time-division-multiplexing. The transmittance over time at each wavelength gives the PPG waveforms for red and infrared wavelengths. Although contact PPG is regarded as a basically non-invasive technique, contact PPG measurement is often experienced as being unpleasant and obtrusive, since the pulse oximeter is directly attached to the subject and any cables limit the freedom to move and might hinder a workflow.
Recently, non-contact, remote PPG (rPPG) devices (also called camera rPPG device herein) for unobtrusive measurements have been introduced. Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g., a camera or a photo detector, can be disposed remotely from the subject of interest. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications. However, remote PPG devices typically achieve a lower signal-to-noise ratio.
Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrates that photoplethysmographic signals can be measured remotely using ambient light and a conventional consumer level video camera, using red, green and blue color channels.
Wieringa, et al., “Contactless Multiple Wavelength Photoplethysmographic Imaging: A First Step Toward “SpO2 Camera” Technology,” Ann. Biomed. Eng. 33, 1034-1041 (2005), discloses a remote PPG system for contactless imaging of arterial oxygen saturation in tissue based upon the measurement of plethysmographic signals at different wavelengths. The system comprises a monochrome CMOS-camera and a light source with LEDs of three different wavelengths. The camera sequentially acquires three movies of the subject at the three different wavelengths. The pulse rate can be determined from a movie at a single wavelength, whereas at least two movies at different wavelengths are required for determining the oxygen saturation. The measurements are performed in a darkroom, using only one wavelength at a time.
Using PPG technology, vital signs can be measured, which are revealed by minute light absorption changes in the skin caused by the pulsating blood volume, i.e. by periodic color changes of the human skin induced by the blood volume pulse. As this signal is very small and hidden in much larger variations due to illumination changes and motion, there is a general interest in improving the fundamentally low signal-to-noise ratio (SNR). There still are demanding situations, with severe motion, challenging environmental illumination conditions, or high required accuracy of the application, where an improved robustness and accuracy of the vital sign measurement devices and methods is required, particularly for the more critical healthcare applications.
Camera-based vital signs measurement in the presence of specular reflectance can cause a lot of errors and it is desirable to use only those regions for measurement that do not contain specular reflectance. Ideally, the negative impact of specular reflectance can be suppressed by using a dedicated illumination and a camera with perfect cross polarization. A photoplethysmograph device applying this approach is disclosed in US 2014/243622 A1. Such ideal system assumes that there are no other sources of (non-polarized) light. However, in practice, it is almost impossible to provide only one perfectly cross-polarized illumination source, since there is always some uncontrolled ambient illumination present.
US 2007/263226 A1 discloses a tissue imaging system for examining the medical condition of tissue having an illumination optical system, which comprises a light source beam shaping optics, and polarizing optics. An optical beam splitter directs illumination light to an imaging sub-system, containing a spatial light modulator array. An objective lens images illumination light from the spatial light modulator array to the tissue. An optical detection system images the spatial light modulator to an optical detector array. In an embodiment crossed polarizers, in particular a drive pre-polarizer in the path of the illumination light and a polarization analyzer in the path of the detected light, are used which are rotated in unison so that their extinction axes rotate into various positions relative to the tissue. By a controller they are rotated by the same angular amount so that they remain crossed to ensure that specularly reflected light and light re-emerging from the tissue while nominally retaining the initial polarization state are both eliminated by the crossed polarization analyzer, whereas light that re-emerges from the tissue with its polarization rotated to some extent by the birefringent structures within the tissue can then have some portion of the light transmitted through the polarization analyzer. In this way, the polarization sensitive optics enable the imaging of the birefringent tissue structures.
US 2013/307950 A1 discloses a method for probing morphology of a tissue surface using a system which may include a light source, a polarizer, an analyzer, and a camera with a plurality of picture elements. The method illuminates the tissue surface with incident light through the polarizer. The camera may capture through the analyzer, scattered light from the tissue surface in a continuous sequence of image frames. Variation of polarization state may be of at least one of the incident light from the light source by varying the polarizer or the scattered light from the tissue surface by varying the analyzer. During the capture, for a picture element of the camera, a varying intensity signal of the scattered light is detected responsive to the varying polarization state.